1. Field of the Invention
The invention concerns satellite navigation systems (of RNSS (Radio Navigation Satellite System) or GNSS (Global Navigation Satellite System) type), and more precisely the generation of messages that relate to such systems and are to be sent to navigation receivers.
Here “satellite navigation system” means any system dedicated to navigation and including a constellation of satellites placed in orbit around a heavenly body (for example the Earth), a set of (terrestrial or spatial) satellite surveillance stations, and a computer center. It could be one of the existing systems (GPS, GLONASS), for example, or one of the planned systems (GALILEO, COMPASS), or one of their equivalents or derivatives.
2. Description of the Prior Art
As the person skilled in the art knows, satellite navigation systems periodically broadcast (navigation and integrity) messages including navigation information relating to the orbital positions and/or synchronizations (internal clock offset relative to the reference clock of the system) of their satellites to navigation receivers of users by means of their satellites (and/or associated equipments, for example pseudolites (such as transmitters on the ground or onboard high-altitude aircraft)).
Such messages generally consist of parameters having their own type and representation (integer, floating point, number of bits and the like) that are predefined and contained in successive data fields associated with each other in a predefined way.
The navigation receivers therefore have available an interface knowing the parameter types and representations that are contained in the broadcast messages and how the fields containing these parameters are associated with each other in broadcast messages. In other words, the interface of each navigation receiver is specifically configured as a function of the format of the messages received in order to be able to understand their contents.
Navigation systems are subject to increasingly frequent changes to implement new services and/or to correct problems. Some of these changes may necessitate modification of the format of the messages. For example, a change may add one or more parameters or modify the type and/or the representation of one or more parameters.
Such format modifications are often incompatible with existing navigation receivers and so certain changes to the navigation systems cannot be put into effect.
For the GPS, it has been proposed to add a new dedicated signal “L1C” to the signal “L1 C/A”. The messages of the new signal L1C are understandable to new generations of navigation receiver and are much more effective than those of the signal L1 C/A. This new signal L1C is transmitted on the same frequency as the signal L1 C/A. The drawback of this solution lies in the fact that it imposes the broadcasting of both signals L1 C/A and L1C, which is costly and consumes too much of the limited resources.
Another solution has been proposed for “augmented” navigation systems or SBAS (Satellite Based Augmentation Systems) that currently have limited capacities that in particular prevent surveillance of more than 51 satellites. It has therefore been proposed to add to the signal “L1” dedicated to messages understandable by older generation navigation receivers a new signal “L5 I” dedicated to messages understandable by new generations of navigation receiver. This new signal L5 I is transmitted on a frequency different from that used for the signal L1. The drawback of this solution lies in the fact that the service offered by the signal L1 is significantly inferior to that offered by the new signal L5 I, which is detrimental to older generation receivers adapted only to the signal L1.
Thus an object of the invention is to remove some or all of the drawbacks referred to above.